This month, the 7 billionth person will be born on a planet already strapped for resources. To mark this extraordinary milestone, TIME explores the most pressing population issues of the day, from the quest for sustainable energy sources to a look at what our biggest cities could look like in the decades to come

9. Shanghai

By Joe JacksonWednesday, Oct. 26, 2011

Carlos Barria / Reuters

Current population: 16.7 million

In some ways, the growth of Shanghai tells the story of contemporary China. In 1980, it had a population of less than 6 million and was not among the top 15 global cities. In 30 years, it has nearly tripled in size to 16.7 million and leaped to the ninth spot on the list. In 2010, the city became the busiest container port the world, surpassing Singapore. It is also a burgeoning financial hub, drawing migrants from across the region. To cope with rapid growth, the government has invested in massive infrastructure projects, including the subway system, which is now the largest in the world.